The present invention relates to temperature self regulating heaters and methods and more particularly to temperature self regulating heaters and methods employing resistance heating in ferromagnetic metals.
Temperature self regulating heaters employing the Curie temperature of a ferromagnetic member as the regulating member are disclosed in a number of patents such as U.S. Pat. Nos. 4,185,632; 4,256,945; 4,745,264; 4,794,226 and others. In U.S. Pat. No. 4,185,632 a constant alternating current at frequencies in the megahertz range is passed in series through a ferromagnetic member which is heated primarily by Joule heating until it approaches its Curie temperature. Below the Curie temperature as a result of skin effect and the high frequencies employed, the current is confined to a narrow region adjacent a surface of the ferromagnetic member. Current density is quite high and I.sup.2 R heating is effective to cause the ferromagnetic member to approach its Curie temperature and become non-magnetic. The current then spreads into the ferromagnetic member which in consequence exhibits a materially reduced resistance to current flow. Since current is held constant and the resistance is materially reduced the temperature falls and an equilibrium temperature is established near the effective Curie temperature and maintained until the heater is subjected to a change in load.
In U.S. Pat. No. 4,256,945, a substrate of non-magnetic, conductive material such as copper is placed in thermal electrical contact with a layer of ferromagnetic material of 1 to 2 skin depths at the frequency of operation, for instance, 13.56 MHz. When the temperature of the device approaches Curie temperature the permeability of the material, .mu., approaches one; and the current spreads primarily into the conductive layer whereby a very material change in resistance is affected. Autoregulation ratios of as high as 160 are available depending upon the initial permeability and resistivity of the ferromagnetic material.
The excitation of such a heater may be accomplished by an induction field excited by a constant current wherein the field is applied to the ferromagnetic material. Such a device is illustrated in FIG. 5 of U.S. Pat. No. 4,745,264. Eddy current and hysteresis losses are induced in the ferromagnetic material producing rapid heating therein. As Curie temperature is approached the coupling is greatly reduced and the current produced in the ferromagnetic layer spreads into a copper substrate further reducing heating. Again, an equilibrium condition is established until the load on the heater is changed.
In U.S. Pat. No. 4,794,226, there is disclosed a Self Regulating Porous Heater wherein a porous ferromagnetic material is dispersed across a path of fluid flow to provide intimate contact between the fluid to be heated and the heater. A flowing fluid heater is also disclosed in FIG. 4 of U.S. Pat. No. 4,256,945 but contact between the fluid and the heated surface is far less than in the porous type heater device. The device in the '945 patent provides far less resistance to flow than the former device but as indicated does not provide the intimate contact between fluid and heater of the porous heater.
It should be noted that the heaters described above are employed to heat solids only on one side whereas in many instances heating from two sides of a body produces more rapid and uniform heating. Additional drawbacks which occur in certain specific applications result from unequal temperature coefficients of expansion of the laminated structures and the inability to change the material of a given Curie temperature for a material of another Curie temperature without changing the entire structure.